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Bulletin of the Museum of Comparative Zoology
AT HARVARD COLLEGE
Vol, LXXII, No. 11
THE OSTEOLOGY OF MESOHIPPUS BARBOURI
By Eeich M. Schlaikjer
With Five Pla
CAMBRIDGE^ MASS., U. S. A.:
PRINTED FOR THE MUSEUM
No. 11. — The Osteology of Mesohipptis barboiiri
By Erich M. Schlaikjer
The subject of this paper is the detailed description of a new and
unusually complete Alcsohippus ^ skeleton from the White River
formation of South Dakota. This skeleton was collected during the
summer of 192S by a fossil-collecting party, under the direction of the
writer, sent out by the IVIuseum of Comparative Zoology at Harvard
To my knowledge there have been twenty species named of the
genus Mesohippus. Many of these species, if not most of them, were
based on very fragmentary material, and it is doubtful if all of them
are valid. This, however, is not a subject to be discussed in these
pages. The only detailed osteological description of MrsoJiippiis is
one by Professor W, B. Scott of Princeton. Until more complete
skeletal material is found, it will be almost impossible to determine
w^hich of the less important osteological characters of Mesohippus
are specific and which are indi\Tdual variations. For this reason it
does not seem advisable to make a detailed comparison of the charac-
ters found in the skeleton described here with the characters found in
M. hairdi. I have chosen to make this paper primarily one of descrip-
tion, and wish to point out that the specimen herein described presents
a number of striking characteristics different from those seen in
ippus as previously known.
A summary of the outstanding characteristics of this skeleton is
as follows : (1 ) strikingly developed alveolar portion of the pre maxillary ;
(2) anterior region of nasals narrow; (3) prominent facial crest on
ary; (4) large, deeply cupped incisors; (5) premolar-molar series
especially long in proportion to skull's length; (6) teeth crowns very
tall; (7) incisor alveolus of mandible broadly expanded; (8) inferior
incisors large and with tall crowns; (9) inferior canine very incisi-
form and set close against I3; (10) inferior premolars and molars
present only minute indication of external cinguH; (11) the posterior
dorsals and the lumbars form no curvature of the vertebral column
when articulated; (12) scapula small; (13) humerus very short in
proportion to length of metacarpal III; (14) metacarpal V a mere
i Proc. N. E. Z06I. Club, 12, 1031, pp. 3o-3G.
392 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY
nodule of bone; (15) lateral and medial digits reduced so they were
non-functional in standing position; (16) femur very short in propor-
tion to length of metatarsal III.
It is evident that this specimen deserved a new specific name, and I
therefore proposed that it be called Mcsohipiyus barboiiri in recogni-
tion of Dr. Thomas Barbour's sincere interest in enlarging the verte-
brate fossil collection at the Museum of Comparative Zoology.
There can be no mistake about the geological horizon in which the
specimen was found. It was collected from the White River formation
in the Big White River Bad Lands, Washington County,
Dakota, and was found in the upper concretionary formation which
lies in the upper middle Oreodon zone.
I owe mv eratitude to Dr. C. C. Mook and to the other members
of the Department of Vertebrate Palaeontology in the American Mu-
seum of Natural History for the permission to study their splendid
horse material; also to Dr. W. K. Gregory of that institution for his
kind and ever helpful suggestions.
Detailed Osteological Description
In general, the portions of the skeleton which have not been pre-
served are the following: the anterior region of the nasals; right I^"^
and left P; the third, fourth and fifth cervicals; the twelfth dorsal;
the posterior part of the sacrum and all the caudals; a large number
of the neural spines ; the pelvis ; most of the left hind limb ; most of the
ribs; and the sternum. All the other bones of the skeleton are beauti-
fully preserved, with practically no distortion.
The dentition of the specimen is unusually well preserved. Incisors
two and three on the left side are present. The others are missing.
They are exceptionally large. The crowns are especially long, are
deeply cupped, and occupy a vertical position. The roots slope up-
ward and backward. The crowns of both canines are missing, but the
prominent roots indicate that they were quite large and were not
erect. P^ is large and has a strong internal cingulum. It was func-
tional. The crowns of P^ to M^ are exceptionally tall. P- is smaller
than any of the remaining teeth. The parastyle is little developed,
though the tooth is not quadrate in outline. The protoloph is con-
tinuous with the ectoloph. The metaconule and the hypocone are
schlaikjer: osteology of mesohippus barbouri 393
united. The internal cingulum is just slightly developed. The anterior
cingulum is minute. The crown is not greatly worn. P^ and P"^ are
very molar-like, and are rather quadrate in outline. There is no indica-
tion of an internal cingulum on these two teeth. The anterior cingulum
is more pronounced on P*^ than on P^. The hypostyle is rather promi-
nent and is somewhat triangular in shape. P^ is distinctly larger than
P^ and sliditlv larger than INI^ M^"^ are rhomboidal in outline.
t> 1/ O
INI^ is less so than the others. There is a prominent oval-shaped
hypostyle on all three molars. There is no indication of an internal
cingulum on M^ and ^I", and there is only a slight indication of one
on jVP. The protoloph and metaloph are rather oblique to the ectoloph
on P^ to M^. The metaloph is not continuous with the ectoloph in any
of the premolars or molars.
The skull is beautifully preserved. There is almost no distortion.
The only missing parts are: right V'^ and left P; the anterior half
of left P^; the posterior half of right P^; a small portion of left P'-; the
anterior part of the nasals; the distal portion of the right paramastoid
process ; the left tympanic bulla ; and a chip is missing from the anterior
of the right facial crest. A portion of the maxillaries and premaxillaries
is chipped away, and a very fine cast of the interior of the skull is
exposed in this region.
The occipital condyles are proportionately narrow and are oblique
in position. The dorsal parts of the articular faces terminate in rather
deep kidney-shaped depressions, forming a sort of constriction just
above the foramen magnum. The foramen magnum is almost circular
in outline, and in the median dorsal position a slight groove extends
upward for a short distance along the external surface of the occipital.
Ventrally the condyles are separated by a narrow, shallow groove.
Near the anterior border of the basi-occipital are two slightly
rugosities separated by the median line. The paramastoid process
is slender^ somewhat compressed and extends quite far below the level
of the condyles. Its position is almost at right angles to the base line
of the skull (a line extending from the optic foramen to the dorsal
border of the foramen magnum) . The distal portion is curved slightly
inward. The supraoccipital is rafher narrow and extends a moderate
distance on the roof of the cranium. A slight median ridge, with a
shallow depression on either side extends along the median line of the
upper half of the supraoccipital. The lambdoid crest is rugose. The
squamosal is large, and extends upward far enough to form the wall of
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the widest portion of the brain. The zygomatic portion extends for-
ward forming a small part of the inferior border of the orbit. The
articular surface of the glenoid cavity is concave from side to side and
convex antero-posteriorly; more so in the central portion than exte-
riorly, where it is broad and somewhat flattened. The post glenoid
process is high and massive. It is somewhat flattened antero-posteri-
orlv, and the intero-anterior portion is articular.
mastoid process is not unusually large. The bulla is not coossi-
fied with the tympanic, but is held firmly to it by the hyoid process
which is deeply imbedded in a groove in the bulla. The anterior border
of the external acoustic meatus is quite heavy.
The basisphenoid is arched posteriorly and becomes flattened as it
extends under the vomer. The pterygoid process extends downward
and forward and is quite pronounced. The alisphenoid extends later-
ally and dorsally from the basisphenoid at a quite gentle slope along
the base of the brain. The anterior portion then bends upward and
forward rather sharply and continues to about half the heiglit of the
In front of the ascending portion of the alisphenoid is the ethmoid
bone. It is narrow and lies in a vertical position. The posterior border
extends the length of the ascending alisphenoid and then continues
upward and forward meeting the anterior border, togetlier with which
it forms a pointed projection under the postorbital process.
The lachrymal bone forms the greater part of the anterior portion
of the orbit. The oval-shaped lachrymal fossa is large and is situated
near the border of the orbit. There is a deep antorbital depression
centrally located on the facial part of the lachrymal.
The jugal bone forms the lower border of the orbit. Superiorly the
maxillo-jugal suture is vertical, and inferiorly it extends downward
and backward. The lower region of the jugal presents a prominent
facial crest which is continuous in front with a similar crest on the
The parietals form the greater portion of the cranial roof. In each
the posterior width is one-fourth narrower than the anterior width.
The sagittal crest is low and in front it separates into two ridge-like
lines which curve outward and forward to meet the frontal crests.
Lying bluntly wedged between the foremost parts of the parietals
are the frontal bones which are prominently convex in this region.
The area just behind the postorbital processes is comparatively wide,
indicating a degree of intelligence which perhaps surpassed that of the
near Oligocene relatives of this animal. The postorbital process is
schlaikjer: osteology of mesohippus barbouri 395
moderately long, extending downward two-thirds the distance to the
zygomatic arch. Just in front of the process is a deep notch which is
preceded by a prominent tubercle. The anterior portion of the frontals
is flattened; and at the median line they extend only slightly between
The nasal bones are triangular in outline. Posteriorly they are
flattened. The anterior portion is tranversely convex and exceptionally
The alveolar portion of the premaxillaries is strikingly developed for
Mesohippus. The transverse measurement between the alveoli of
incisors three is 22 mm. The vertical diameter is also comparatively
large. At the median line there is evidence of a foramen incisivum.
The diastema between incisor three and the canine measures one-
fourth the distance of the canine-premolar diastema. The ascending
portion of the premaxillary is quite oblique in position. The upper-
most portion is missing and in restoring I have tried to follow out the
curvature of the parts that have been preserved. The position of the
premaxillaries and the nasals seems to indicate that the anterior nares
w^ere quite high, and were broader below^ than above. It is difficult to
determine the exact extent of the palatine plates of the premaxillaries
as they are not entirely preserved.
The facial portion of the maxillary is somewhat convex
rather concave above; particularly in the anterior region.
lary portion of the facial crest stands out considerably from the tooth
row and lies above the posterior half of the first molar. It has the form
of a prominent elongated tubercle. The infraorbital foramen is situated
twenty -six millimeters in front of the orbit and eighteen millimeters
above the front of premolar four. The diastema between the canine
and the. first premolar is unusually short. There is a rather abrupt
constriction of the maxillarics at about the middle of the diastema,
although this region is comparatively broad. The palatine surface is
transversely convex and is slightly raised along the median suture.
The alveolar tuberosity extends a good distance behind the last molar
and is sc^uare in outline.
The palatines form the anterior margin and the lateral walls of the
posterior nares. They do not greatly constrict the opening posteriorly.
The inferior portion of the wall ot the orbit is formed by the palatine
bone and it extends forward to make contact with the lachrymal.
The vomer is long and narrow and is slightly constricted where it
articulates with the pterygoids.
396 bulletin: museum of comparative zoology
The Sktdl in General
The broad diameter across the molar region, the uniformity in width
throughout the length of the zygomatic arches, and the compara-
tively expanded cerebral region give the skull a broad massive appear-
ance. The area anterior to the frontals is constricted though it is
relatively long. The great development of the pre maxillary alveolar
region is certainly an advanced characteristic and is unusual in the
genus MesoMpjms. It is therefore one of the striking features of the
skull. The large cupped incisors, the tall crowns of the premolars
and molars, and the comparatively long tooth series are additional
characteristics of the skull showing advancement at this stage of the
evolution of the horse.
Right Pi and P2, and the anterior portion of left P3 are missing;
otherv\ise the lower dentition is complete. The incisors are very large
and the crowns are exceptionally long. I3 is slightly smaller then Ii
and I2. Each has an internal ^^cingule" (a term that I suggest to de-
scribe a flattened cone-like cusp occupying the position of a cingulum)
which is situated outwardly from the median line. A quite prominent
ridge extends from the outer rim down to the inner base of the cingule.
There is a pocket-like depression on either side of the ridge. The
outer depression is slightly deeper than the inner onie. These depres-
sions are deepest in I3.
The canine is set closely against 1 3 and is very incisiform. A rather
heavy ridge extends from the outer rim to the internal base, and has
a slight depression on each side. The posterior depression is a little
more pronounced than the anterior one.
Pi is minute and is set close to P2. The tooth is compressed laterally.
There is a shallow depression in the anterior internal portion of the
crown, and the posterior portion presents a V-shaped ravine at the base
of which is a well defined entoconid. The tooth, therefore, has practi-
cally the same form as P2 though it is very much smaller. It was not
functional at the time of the animal's death. P2 is incompletely molar-
form, the metaconid not being developed. P4 is the largest tooth of
the Pi — IMo series. M3 has a well developed, deeply cupped hypo-
conulid, and is therefore the longest of any of the inferior teeth. P4
has the greatest breadth. There is only the very slightest indication
of an external cingulum on any of the inferior molars.
schlaikjer: osteology of mesohippus barbouri 397
The horizontal ramus is elongated. It tapers forward and is bicon-
vex in transverse vertical section. The two rami are firmly fused at
the symphysis, which is somewhat shortened. There is a rather sharp
constriction at the diastema, but this region is comparatively broad.
The incisor alveolus is broadly expanded. The symphysial region is
extended upward and forward at a rather sharp angle from the remain-
ing portion of the horizontal ramus. This shortening and angulation
of the symphysial region was probably necessary to supply additional
strength for the support of the large incisors. The ventral border of
the ramus posterior to the symphysis is nearly straight . At its posterior
part the vascular impression is fairly well marked. The angle is
rounded. Its posterior border is thickened and slightly medially
curved. The foramen mandibulare is large and is situated on a level
with the base of the tooth row. The condvle is considerablv elevated
above the level of the molars. It is extended transversely and occupies
an oblique position, the inner portion being directed backward and
nward. The coronoid process extends high above the condyle. It
is thin transverselv, curved backward and slightlv inward.
Condylo-basal length 176.
Zygomatic breadth 85.
Greatest breadth across squamosals on molar arches .... 82.
Greatest breadth across squamosals on cranium 52.
Breadth on upper rim of post glenoid notch 51.5
Interorbital breadth across frontals 45.
Distance from anterior rim of orbit to pmx. (I alveoli) . . , . 88.
Distance from anterior rim of orbit to supraorbital crest . . . 110.
Distance from mandibular condyle to pmx 146.
Breadth of muzzle at anterior root of M^ 60.5
Distance from anterior end of internal nares to pmx. (I alveoli) SO.,
Distance from anterior end of internal nares to condvles . . . 96.
Greatest width of condvles ..." 30.5
Distance from ventral portion of exocc. to top of Supraocc. . . 48.
Width of premaxillaries at posterior of P alveoli
Breadth across P^ to P^ at center of crown .
Breadth across P'* to P^ at center of crown 57.
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Breadth across M^ to M^ at center of crowni
Breadth across ]\P to j\P at center of crown
Length of diastema (P to P^) external . . .
Molar series on alveoli (P^ to IVP) external .
Distance from P" to P^ on alveoli (external)
Distance from M^ to ]VP on alveoli (external)
Height of crown of P at center of tooth (external)
Lateral distance across P .....
Fore and aft distance at center of I-
Height of crown of P
Lateral distance across P
Fore and aft distance at center of P
Height of crown of P^ at highest point (external)
Height of crown of P- on paracone (external)
of P- at center of crown
Length of P- at center of crown
Height of crown of P^ on paracone (external)
Height of crown of P^, IM^' ^' ^ on paracone (extern
Breadth of P^ at center of crown .......
Length of P^ at center of crown .......
Breadth of ]M^ at center of crown .......
Length of INI^ at center of crown
Breadth of ^P at center of crown
Length of ]\P at center of crown
Greatest breadth of
Greatest length of ]NP
The Vcrtchral Column
Atlas: This vertebra is broad in proportion to its length. The
anterior articular cavities are deep. They almost meet ventrally, but
dorsally they are widely separated by a broad notch. The neural arch
is broad and high. Medially there is a prominent, sharply convex
region. There is only a slight indication of a ridge above the inter-
vertebral foramen. The ventral arch is more massive, narrower, and
shorter than the dorsal arch. The surface is flattened. The hypa-
pophysial tubercle is posterior in position. It is fairly large and very
prominent. The posterior articular faces are high. They are widely
separated al)Ove and are nearly confluent below. The surfaces are
slightly convex and the inner borders are rounded off. The wing is
schlaikjer: osteology of mesohippus barbouri
somewhat short, but fore and aft it is quite extended. The dorsal
surface is sUghtly concave. At the anterior base is the spinal nerve
notch which is fairly deep. The foramen trans versarium is situated
on the border of the posterior root of the wing where it meets the
articular face. The fossa atlantis is large, rounded and deep. The
posterior border is narrow.
Axis: Only the anterior region of the axis is preserved. The odontoid
process is broad and is turned upward at the end. It has a low convex
ventral surface; the dorsal surface presents a fairly prominent median
ridge with a depression on either side. The anterior articular processes
are low, broad, and are confluent with the lateral margins of the
odontoid process. The articular surfaces are slightly concave trans-
versely and markedly convex dorso-ventrally.
CcTvicah: The third, fourth, and fifth cervicals are absent. The
sixth cervical vertebra is deep and massive. The neural spine is in-
complete, but the portion present indicates that it was scarcely more
than a prominent ridge. The prezygapophyses are missing. The post-
zygapophyses project slightly behind the neural spine. The transverse
process is straight, short, and heavy. The front part of the inferior
lamella is not preserved. Posteriorly the lamella extends far downward
and slightly outward; its inferior border is flattened. Below the pos-
terior end of the foramen transversarium is a rather large fossa. The
head of the centrum is oval-shaped; it is wider and more flattened
above than below.
The seventh cervical is considerably shorter, but is wider than the
sixth. The neural spine is very short, measuring only eighteen milli-
meters above the dorsal border of the neural canal. The prezygapo-
physes are wider than the postzygapophyses; the latter extend slightly
behind the neural spine. The transverse process is moderately long.
The distal border is bent upward and is thickened; anteriorly it is
bent downward and is thin. The posterior part of the centrum is wide
Dorsals: The exact number of dorsal vertebrae is uncertain. The
matrix containing the specimen w^as broken just behind the eleventh
dorsal. I have reckoned the twelfth to be missing. The remaining five
were in continuous series; thus making seventeen dorsals in the
mounted specimen. The first of" the series resembles the seventh
cervical in some respects. The centrum is flattened, though it is
shorter and the median ventral portion is heavier. The transverse
process is even longer and is formed into a deep semi-circular facet
for the tubercle of the first rib. The prezygapophyses are wnder than
400 bulletin: museum of comparative zoology
the postzygapopliyses, and the latter extend only slightly behind the
neural spine. The neural spine is thin in front and broad behind; gi\Hng
it a triangular shape when viewed from above. There is a median
ridge on its posterior border. The succeeding dorsals show modifica-
tions of these characters. The neural spine is heaviest on the second
; from there on back to the thirteenth it gradually becomes
Hghter. Only a small part is missing from the end of the spine of the
second dorsal. The tips are missing from the next four, and the greater
portion of each spine of the others has been lost. In restoring the
missing ones I have been guided only by what remains of each. The
semi-circular facets on the transverse processes are gradually reduced
so that they are flattened on the sixth to the last. The zygapophyses
on the third to the sixteenth are developed on the neural arch. The
last dorsal is the only one which has a median ventral keel; the keel
shows greater development in front than behind. The vertebra also
presents a quite prominent elongated tubercle on the side of the
centrum behind the rib facet. From the fifteenth to the seventeenth
the length of the centrum gradually increases.
Lumhars: The lumbar vertebrae Avere discontinuous behind the
third of the series. Two lumbar centra were associated with the speci-
men, and I have considered them to be the fourth and fifth. The
sixth and seventh w^ere continuous with the sacrum. All of the lumbars
are fairly completely preserved. A large amount of each neural spine
is preserved, except on four and five w^here only the centra remain.
The seventh is the only one which retains the greater portion of its
transverse processes. The centra of the first four are quite elliptical in
cross-section and have a distinct ventral crest. From the fifth to the
last the centra become more flattened and the ventral crest disappears.
The basal portions preserved of the transverse processes indicate that
they were moderately massive and that they extended outward and
slightly downward. On the last three they project moderately forward.
The transverse process of the last lumbar has a thickened oval-shaped
facet for articulation with the wing of the sacrum.
Sacnwi: Only the first of the sacral vertebrae is preserved. The
centrum is considerably flattened. The wing is massive and curves
slightly dow^nw^ard. On its anterior border is a convex surface which
fits into the facet on the transverse process of the last lumbar. On the
postero-dorsal surface of the w^ing is the elongated, concave, rugose,
articular surface for articulation with the ilium,
Candals: None of the caudal vertebrae are preserv
SCHLAIKJER: osteology of MESOHIPPUS BARBOUR!
Vertebral Column in General
The vertebral column presents a number of interesting features,
such as: the shape of the neural arch of the atlas and the position of
the foramen transversarium on that vertebra; the shortness of the
neural spines on the sixth and seventh cervicals; the absence of a
ventral keel on all the dorsals except the last; and many other similar
characters. But perhaps most striking of all is the lack of curvature
of the column when the vertebrae are articulated. The posterior
cervicals and the anterior dorsals form a downward curving of the
column but the posterior dorsals together with the lumbars almost
form a horizontal line. This certainly seems to be an advanced char-
Most of the ribs are lacking. The first three and the fifth on the left
side and the second and third on the right were found in position and
are complete. They are comparatively short; the first is rounded and
slender, but the others are broad and very flattened. The proximal
ends and various parts of a number of others are present. None of the
sternum was preserved but the increase in length of the first five ribs
seems to indicate that it must have occupied a position oblique to the
long axis of the horse^s body.
Bones of the Fore Limh
The greater portion of the left scapula and a good deal of the right
is preserved; all the remaining bones of the fore limbs, including a
number of the sesamoids, are complete and Avere articulated.
Scapula: The glenoid cavity is slightly longer than broad, and it is
fairly shallow. The coracoid process is large and massive; it is some-
what recurved at the distal end. The neck is long and exceptionally
slender. The anterior border is moderatelv thick near the coracoid
process. The posterior border on the lower half of the scapula is con-
siderably thickened. The distal part of the scapula is erect, but the
proximal region is curved in, giving it the appearance of a bow when
viewed from behind or in front. The spine is tall and is bent sharply
Humerus: This bone is proportionately very short for Mcsohippns.
The head is large, strongly convex and is rather circular in outline.
The external margin is considerably lower than the internal. This,
along with the strong convexity of the surface, perhaps indicates that
402 bulletin: museum of comparative zoology
there was a good deal of lateral swing to the foreUmb during locomo-
tion. The anterior portion is large and overhangs the l^icipital groove.
The more lateral part is well developed and its outer surface presents
a large rugose triangular area for the attachment of the infraspinatus
muscle. At the bottom of the bicipital groove there is a slight mdica-
tion of a tubercle. The inner tuberosity is large and has a flattened
dorsal surface. The shaft is comparatively slender and is twisted; it is
irregularly oval-shaped. The deltoid tuberosity is prominent and
rugose. The distal end is somewhat broadly expanded; the trochlea
is oblique to the long axis of the shaft and the epicondyles are fairly
Radius: The radius is comparatively long. The shaft is gently curved
forward and is flattened antero-posteriorly; more so medially than
laterally. It is rather expanded at both ends. The interior facet of
the proximal end is fairly shallow and is larger than the narrow median
facet. The external facet is large and flattened; it extends outward and
upward. The distal end is massive. The grooves for the common and
lateral extensor tendons are well marked. On the inner anterior surface
there is a shallow oblique groove for the tendon of the extensor carpi
obliquus muscle. The scaphoid facet is quadrilateral and is concavo-
convex from before backwards. The lunar facet is similar in shape,
though with less convexity and with more transverse expansion.
Ulna: The ulna is much reduced. The olecranon is high and massive.
The medial side is concave, the lateral is convex. The top presents a
heavy rounded tubercle situated posteriorly. There is a prominent
ridge extending from the tubercle with a fairly large, flattened face
. The processus anconaeus is very prominent. The semi-
lunar notch is rather deep and its surface is convex transversely. The
distal end of the ulna is not fused with the radius. Its articular face
is convex and bears a postero-internal groove.
Carpus: The scaphoid is deep, somewhat laterally compressed and
is rather six-sided in appearance. The anterior portion of the dorsal
surface is convex and the posterior is concave. There is an upper and a
lower facet for the articulation with the lunar. On the distal surface
the magnum articulation is slightly convex. Directly behind is the
facet for the trapezoid which is rounded and concave. There is a
minute facet behind this for the trapezium.
The lunar is proportionally long; it is wider in front than behind.
The dorsal surface is saddle-shaped. Laterally the lunar is in contact
with the scaphoid at the superior and inferior margins. It is in con-
tinuous contact with the cuneiform. The ventral surface is narrower
schlaikjer: osteology of mesohippus barbouri 403
than the dorsal. The articulation with the magnum is only slightly
wider than the unciform articulation. There is a tubercle on the
The cuneiform is narrow above and broad below. On the anterior
internal proximal surface there is a small triangular facet for the
radius. The ulna facet is concave and is higher in front than behind.
On the distal surface the unciform is concave and is triangular in
outline. The pisiform facet is oblique in position.
The pisiform is deep, somewhat flattened and moderately long. The
internal surface is concave and smooth. The external surface is convex
and rugose. Its anterior part bears a shallow oblique groove for the
long tendon of the ulnaris lateralis. The facet for the ulna is round and
convex. The cuneiform facet is antero-inferiorly situated. It is elon-
gated and convex, and is separated from the ulna facet.
The trapezium is not preserved.
The trapezoid is low and rather wedge-shaped, behig narrow in
front and broad behind. The proximal surface is convex and continu-
ous on the volar surface. The distal surface is flattened. The articula-
tion for metacarpal II is rather triangular in outline. There is a narrow
elongated articular surface on the ventral border for articulation with
The magnum is the broadest of any of the distal carpal bones. On
the proximal surface, the facet for the scaphoid is twice as broad as
the facet for the lunar. The distal surface is broadly triangular in
outline and is almost flat.
The unciform is somewhat shortened, its greatest length being only
one-fourth greater than that of the magnum. It is rather broad. On
the proximal surface the lunar facet is fairly large and has a dorsal
position. The cuneiform is slightly convex. On the distal sur-
face, the facet for metacarpal IV is slightly concavo-convex from
before backwards. The facet for metacarpal III is ventro-laterally
situated. There is a small facet for metacarpal V.
Metacarpus: Metacarpal III is long in proportion to the other bones
of the limb. The shaft is rather semi-elliptical. The anterior surface is
transversely convex. The posterior surface is slightly convex from
side to side and there is a medial shallow groove running longitudinally.
The proximal surface is flattened ^anteriorly, and it bends down to a
certain extent posteriorly. The unciform facet is oblique in position.
The distal end is expanded and thickened. The carina is prominent
and extends slightly anterior to the center of the long axis of the bone.
On either side of the articular surface is a small fossa above which is a
404 bulletin: museum of comparative zoology
The lateral metacarpals are shorter, thinner, and more compressed
than metacarpal III. Metacarpal IV is the shorter of the two. The
distal extremities are rounded and the carinae are mostly confined to
Metacarpal V is minutely represented — its greatest length being
only eleven millimeters. The head is half the size of that of metacarpal
IV. The entire metacarpal amounts to nothing more than a nodule of
Phalanges: The phalanges of the lateral digits are short and stubby;
the unguals are short, narrow, and pointed. Those of the middle digit
are very much larger. The first phalanx is long, broad and thickened
above, and more narrow and compressed below. The second phalanx is
much broader than it is long. It is not quite two-thirds as long as the
first and is much compressed antero-posteriorly. The ungual phalanx
is somewhat longer than it is broad. The dorsal surface is strongly
convex. The ventral surface is prominent but not greatly extended
posteriorly. The distal border is notched and a V-shaped groove
extends upward from the notch a short distance along the dorsal
Length of scapula
Fore and aft diameter of glenoid cavity 19.5
Transverse diameter of glenoid cavity 17.
Distance from coronoid process to posterior rim of glenoid
Narrowest fore and aft distance of neck of scapula 13.5
Length of humerus, external tubercle to posterior trochlea . . 123.
Length of humerus, head to posterior trochlea 117.
Shortest width of shaft
Shortest fore and aft distance of shaft 14.
W idth of distal end of humerus on lateral ligament depressions . 23.5
Greatest length of ulna . . .
Length of radius on inner side
Greatestwidthof proximal end of radius 24.5
Greatest width of distal end of radius 21.
Width at center of radius shaft
Greatest height of lunar . . .
Greatest height of magnum .
Width of magnum (anterior) .
schlaikjer: osteology of mesohippus barbouri
Greatest length of metacarpal III .
Greatest width at proximal end of metacr4rpal III
Greatest width at distal end of metacarpal III .
Greatest length of metacarpal II
Greatest length of metacarpal IV
Greatest length of metacarpal V .......
Lateral median length of first phalanx, 3d digit .
Lateral median length of second phalanx, 3d digit
Median dorsal length of ungual phalanx, 3d digit .
Width of ungual phalanx (across angles) 3d digit .
Bones of the Hind Limb
Practically all of the right hind limb is preserved. The pelvis and the
left hind limb, with the exception of a part of the proximal end of the
femur and most of the astragalus, are missing.
Fcmvr: In proportion to the length of the metatarsals the femur is
very short. The proximal end is large. The head is practically hemi-
spherical and is directed upward, forward and inward. The notch for
the accessory and round Hgaments is deep and broad. The great tro-
chanter is very massive. The anterior part is low and rugose; a shallow^
notch separates it from the posterior part which stands high above the
head and is mediallv curved at the end. The trochanteric fossa is
elongated and deep. The second trochanter is prominent; the distal
end of the third is missing but the region preserved indicates that it
was weW developed and was curved forward.
The shaft is stout. Just below the third trochanter it is laterally
compressed and deep; it then becomes circular, and expands and
deepens at the distal extremity.
The distal end is expanded both transversely and antero-posteriorly.
The trochlea is broad and slightly oblique to the long axis of the shaft;
the medial ridge is heavier and more extensive than the lateral. The
condyles are large and extend rather far posteriorly; the lateral is
much larger than the medial. The medial and lateral epicondyles are
fairly prominent; the supracondyloid fossa is elongated and deep.
Tibia: This bone is long and rather slender. The proximal end is.
large and triangular in general outline when viewed from above..
The condyles are saddle-shaped, and the lateral is broader than the
medial. The intercondyloid eminence is prominent; the popliteal notch
is broad and fairly deep. The sulcus muscularis is semicircular and
406 bulletin: museum of comparative zoology
deep. The crest is very prominent and rugose, and occupies the upper
third of the shaft. The distal end is narrow but massive; the medial
portion is larger than the lateral. The articular facets for the astrag-
alus are oblique to the long axis; the medial is narrower and deeper
than the lateral.
Fibula: Both ends and a part of the distal region of the shaft are
preserved. The proximal end is large; it is flattened transversely, and
the anterior and posterior borders are rounded. The distal end and a
part of the lower end of the shaft are coossified with the tibia. The end
is expanded and the calcanear facet is large and flattened.
Patella: The patella is cubical in outline with a pointed postero-
distal end. The anterior surface is strongly convex and rugose; the
posterior presents two articular surfaces, of which the medial is slightly
larger. The medial border is short and slightly convex; the lateral
border is long and straight. The base is transversely convex and is
slightly concave in the antero-posterior diameter.
Tarsus: The calcaneum is proportionately short; it is transversely
compressed. The distal end of the body is thickened, rounded and
rugose. The plantar border is straight and slightly expanded at each
end. The posterior half of the dorsal border is straight. The cochlear Is
process is prominent and is expanded laterally. The sustentaculum is
large and massive. The cuboid facet is somewhat narrowed and is
The trochlea of the astragalus is oblique in position; the groove is
narrow and deep. The distal surface is transversely concave and is
convex antero-posteriorly. There is a small cuV)oid facet. The distal
part of the medial surface bears a large tuberosity.
The cuboid is deep, transversely compressed and is not in contact
with metatarsal III. The navicular is low and broad. The meso- and
ento-cuneiforms are coalesced. The en to-cuneiform is rounded pos-
teriorly and is in contact with the cuboid; it is deeper than the meso-
cuneiform. The ento-cuneiform is broad and rather low.
Metatarsus: Metatai'sal III is proportionately long and fairly stout.
The proximal end is narrow and deep and does not come into contact
with the cuboid. The distal end is broadly expanded but massive.
On the dorsal side above the articular surface is a transverselv elon-
gated shallow^ pit. The lateral tubercles are very prominent. Meta-
tarsals II and IV are much shorter than metatarsal III and are much
lighter in construction.
Phalanges: The phalanges of the hind foot are practically the same
as those of the fore foot except they are more massive; the portions
schlatkjer: osteology of mesohippus barbouri 407
preserved of the lateral and medial digits seem to indicate that they
were shghtly shorter.
Greatest length of femur
Length of femur from head to internal trochlear ridge .... 157.
Length of femur, center of 3d trochanter, to external trochlear
Shortest transverse measurement of femur shaft 13.
Distance across condyles of femur 33.
Length of tibia from medial condyle to internal malleolus . . 182.
X-^Cll^til 01 TlDllicl - ♦..•...*
Metatarsal III (maximum length) 117.
Maximum width, proximally, of metatarsal III 13.
Maximum width, distally, of metatarsal III 15.
Maximum length of metatarsal II
Maximum length of metatarsal IV
Length of 1st phalanx, 3d digit
Lengthof 2d phalanx, 3d digit, at center of side 10.
Lengthof ungual phalanx, 3d digit, dorsal 19.
llie Limbs in General
Perhaps the most striking feature of the limbs is the shortness of
the proximal elements as compared with the proportionately long
metacarpals and metatarsals. It is a well known fact that lengthening
of both proximal and distal limb elements creates a long stride in
locomotion but retards speed. On the other hand, if only the distal
elements of the limb are lengthened and if the proximal elements are
shortened then both the stride and the motion of the limb are increased,
thus giving the animal greater speed. According to the accompanymg
table, showing the metacarpo-humerus and metatarso-femur ratios,
it seems evident that this specimen was proportionately more swift
in running than were his near Oligocene relatives. It is indeed inter-
esting to note that the speed of this little horse was, proportionately,
perhaps even greater than that of the upper Oligocene ^Vstilt-footed''
bulletin: museum of comparative zoology
Table showing a comparison of limb ratios of M. barbouri with
those of other species of Mesohipjms,
M. C. Z.
M. bairdi \
A. M. N. H.
Another striking feature of the Hmbs is the reduction of the lateral
and medial digits so* they were non-functional in standing position.
This does not mean, however, that they were entirely useless. They
were probably still used to help keep the feet of the animal from
miring deeply into the marshy lands on which it probably spent a
good deal of its roving existence.
It is hoped that what has been written in the foregoing pages wi
contribute something to that which is already known of Mesohipinis.
To say what w^as the exact ancestry of M. barbouri would only be
hypothetical. I believe, however, that this species represents a pro-
gressive line of three-toed horses which had its origin from some as
yet undiscovered genus of horse in middle or late Eocene times; and
from a species more advanced than that which gave rise to the line
of Oligocene horses represented by M. bairdi.
schlaikjer: osteology of mesohippus barbouri 409
1903. New vertebrates from the Montana Tertiary. Ann. Carn. Mus,,
2, pp. 145-200.
Fare, M. S.
1896. Notes on the Obteology of the White River horses. Proc. Amer.
Phil. Soc, May 15, 35, pp. 147-175.
Lambe, L. M.
1905. Fossil horses of the Oligocene of the Cypress Hills, Assiniboia.
Trans. Royal Soc. of Can., Second Ser., 11, Sec. IV, pp. 43-52,
1905. On the tooth structure of Mesohippus westoni (Cope). Amer.
Geol., 35, April, pp. 243-245.
1869. Extinct Mammalian Fauna of Dakota and Nebraska. Jour.
Acad. Nat. Sci,, Phila., 2d ser., 7, pp. 257-330.
Matthew, W. D.
1903. Evolution of the horse. Guide Leaflet No. 9, Amer. Mus. Jour.,
3, No. 1, Second ed., 1905.
1915. Chmate and evolution. Ann. N. Y. Acad. Sci., 24, pp. 171-318.
33 text figures.
1926. The evolution of the horse. A record and its interpretation.
Quart. Review of Biol., 1, No. 2, pp. 139-185. 27 text figures.
1928. Outline and general principles of the history of life. Univ. of Cal.
Press. Berkeley, Cal. 253 pages, 21 text figures. (Chapter XX.
The evolution of the horse, pp. 160-177).
Matthew^, W. D. and Chubb, S. H.
1913. Evolution of the horse. Guide leaflet No. 36. Amer. Mus. Nat.
Hist. Pt. I, pp. 1-35 by W. D. Matthew, evolution of the horse in
nature. Pt. 2, pp. 39-63 by S. H. Chubb, the horse under domesti-
cation. Republished 1921 with slight revision.
OSBORN, H. F.
1904. New Oligocene horses. Bull. Amer. Mus. Nat. Hist., 20, pp. 167-
179, pis. Ill, V.
1918. Equidae of the Oligocene, Miocene, and Pliocene of North America.
Iconographic revision. Mem. Amer. Mus. Nat. Hist., n.s., 2,
pp. 1-217, pi. 1-54, and 173 text figures.
gCHLAIKJER, E. M.
1931. Description of a new Mesohippus from the White River formation
of South Dakota. Proc. N. England Zool. Club, 12, pp. 35-36.
Scott, W. B.
1891. On the Osteology of Mcsohippvs and Leptomerijx, Jour. Morphol.,
6, pp. 301-400.
410 bulletin: museum of comparative zoology
SiXCLAIR, W. J.
, The faunas of the concretionary zones of the Oreodon beds, White
River Oligocene. Proc. Amer, Phil. Soc, 63, No. 1, pp. 94-133.
pL 1 and 10 text figures.
1925. The mounted skeleton of a new Mesohipjms from the Protoceras
beds. Proc. Amer. Phil. Soc, 64, pp. 55-63, ph 3.
EXPLANATION OF PLATES
ScHLAiKJER.— The Osteology of Mesohippus barbouri
BULL. MUS. COMP. ZOOL.
SCHLAIKJER. MESOHIPPUS. PLATE 1
SCHLAIKJEB.— The Osteology of Mesohippus barbouri
Mesohippus harbouri Schlaikjer
Skull and jaws, left side; Actual size.
BULL. MUS. COMP. ZOOL.
SCHLAIKJER. MESOHIPPUS. PLATE 2
ScHLAiKJEB. — The Osteology of Mesohippus barbouri
Mesohippus barbouri Schlaikjer
Skull and jaws, palatal view. Two-thirds actual size.
BULL. MUS. COMP. ZOOL.
SCHLAIKJER. MESOHIPPUS. PlATE 3
ScHLAiKJER. — The Osteology of Mesohippus harbour i
Mesohippus harbotiri Schlaikjer
Vertebral column; Fig. 1, left side; fig. 2, ventral view; fig. 3, dorsal view;
Reduced 4f times.
BULL. MUS. COMP. ZOOL.
SCHLAIKJER. MESOHIPPUS. PLATE 4
ScBXAiEjEB. — The Osteology of Mesohippus barbouri
Mesohippus barbouri Schlaikjer
Mounted skeleton, right side. ^ natural size.
Photographs by Irving Dutcher.
Skeleton mounted by Charles J. Lang.
All figures based on the type, M. C. Z. no. 17,641.
BULL. MUS. COMP. 200L.
SCHLAIKJER. MESOHIPPUS. PLATE 5